Neonatal platelets do not readily associate with granulocytes in vivo or in vitro. (A) Significantly fewer endogenous neutrophil-platelet aggregates are found in the circulation of neonates compared with adults. n = 5. Significance determined using unpaired Student t test P < .05. Leukocytes were defined as CD45⁺, granulocytes were defined as CD45⁺/GR-1⁺ double-positive, and platelets were CD41⁺. (B) Representative images of neutrophil-platelet rosettes with 1, 3, and 5 platelet spots. CD41 PE is shown in yellow, and GR-1 fluorescein isothiocyanate is shown in purple. Images were collected at 40× magnification. Scale bar, 10 μm. (C) Fewer rosettes were formed when neutrophils were mixed with activated platelets from P1 pups (+P1) when compared with rosette formation with activated platelets from adults (+Adult). Platelets were activated with 0.5 units/mL thrombin before mixing in a 10:1 ratio with bone marrow-enriched neutrophils. Rosettes were defined as a single neutrophil with 2 or more platelets attached. Significance was determined using an unpaired 2-tailed Student t test *P < .05; n = 3.